My husband and I were pondering this one yesterday. Having studied Hebrew for five years, I admit anything else looks very easy to me. It seems to me that a tonal language such as Chinese must be very hard to learn whereas a romance language such as Spanish is far easier given the many cognates. My Russian next door neighbor tells me English and Russian have very little in common.
This has been the subject of much research. The answers are Japanese followed by Russian.
Unfamiliar sounds are a matter of getting used to them. But complex grammar is always a challenge.
Wasn’t the reason that Daniel Tammet was challenged to learn Icelandic in a week because it was said to be notoriously difficult for English speakers to learn?
ETA: LavenderBlue - did you learn Hebrew without the “dots”? That would be hard.
Most of the Hebrew I learned was conversational and based on basic rote memorization of davening (praying), brachot (prayers) and zimrot (songs). My brain rebelled against the written language so I basically ignored it. I know the aleph bet and what sounds the letters signify as well as how to write them in Hebrew script. I also know the spelling of about 500 basic Hebrew words but that’s about it. I really can’t read or write Hebrew in any meaningful sense.
I dot know how it ranks against Icelandic, but Irish Gaelic has thoroughly kicked my behind.
Anecdotal: My father was an ESL teacher, living in the countries where he taught English, for his entire life. He maintained that Hungarian was the hardest language for him to learn (he lived there for three years.)
I will say that different languages offer different challenges. French was relatively easy to learn to read. Learning to hear it has defeated me. I can (or could anyway) speak it in simple conversations. In German, hearing is much less of a problem. The grammar is relatively simple for an English speaker to catch onto. The vocabulary, once you get past the very basic language (where English has retained a lot of the old Germanic words) is somewhat daunting, but learnable.
As for hardest, well the tonal languages are hard, but my understanding is that Chinese grammar is simple. No inflections, not even plurals. No tenses; you have to express temporal information explicitly, with words or phrases (yesterday, some time in the past, a long time ago, etc.)
As for “easiest”, that’s surely going to depend on where you draw the line on what counts as a “foreign language”. Does Braids count? How about Middle English? Texan? Legalese? On the other hand, some folks would probably find it easier to learn, say, Frisian than Legalese.
Tones are not that hard. I can teach the basics in 15 minutes. It’s not too completely different than how emphasis can change meaning in English.
Remembering tones and using them consistently is a bit of a pain, but you don’t necessarily have to get them perfect. I was TERRIBLE with my tones and was fine at making myself understood.
Chinese must have plenty of pun potential.
Navajo is nearly impossible. I think that maybe there is no historical record of any non-native speaker of Navajo ever becoming reasonably fluent.
Cite? I have never seen anything so clear cut. I would love to see this entire body of research which I’ve missed in my 30 years of learning foreign languages.
Silly me. All this time, I’ve been relying on the Foreign Service Institute on their rankings. Come to think of it, they must be pretty red faced as well. Perhaps you can send your cites to them as well.
All snarks aside, the GQ answer is that in the FSI’s opinion, the most difficult languages they teach are
Russian is listed as a Category IV.
It must be noted that (unless there is a field of research showing differently, for which we await the cite) this is strictly a discussion of the difficulties in general and that individuals’ aptitudes for learning the various languages will vary, sometime considerably. For example, I had a relatively easy time learning the Japanese kanji characters where some friends found that to be extremely difficult.
Personally, I rank Mandarin much more difficult than Japanese, but that is probably because learning Japanese grammar is more conceptual, which is my strength, where Chinese is tonal and has many sounds not in English, requiring learning by ear, which is my weakness.
The difficulty of a definitive answer to this OP question, is that some languages are laughably easy to learn to make simple conversations, but extremely difficult to master, such as Mandarin. Does that make Mandarin an easy language, or a hard one?
I think Swahili and Malayan/Bihasa Indonesia are reputed to be fairly easy languages to learn fairly well, both of them being fairly newly constructed languages which are not burdened by a lot of archaisms of grammar, and using mainly flat simple phonemes that are common to European languages…
And as TokyoBayer points out, some people learn languages by sight and some by sound, and that inclination would make certin languages easy for one person and hard for another. For a person who learns by reading, a language that uses the Roman alphabet has a great advantage starting out. For one who learns best by ear, that wouldn’t be a factor.
That’s one hell of a fascinating link, Tokyo. Thanks!
I think Hebrew spans the hard to read and hard to speak category. The alphabet is entirely different, written from right to left and contains the formal printed script and the casual handwritten version. It also has some sounds like ch that can be hard to master correctly. Also, gender, gender freaking gender on everything. And then of course that fact that in Hebrew he means she and who means he. 
My brain was irked.
And me means who.
English is the same way. It is very easy for even poor speakers to make themselves understood as demonstrated by the sheer numbers of people you can use it as a second language all around the world in lots of different variations and even pidgin versions. Sure, media exposure helps with that but it is also an easy language to make yourself understood no matter how badly you screw it up. All you have to do is string some reasonable facsimile of English type words in a some semi-random order together and people can generally understand you to some degree.
However, English is also very difficult to get completely correct as well because it has so many irregular spellings and verb conjugations. Many native speakers still haven’t ever gotten much of it completely right 
I am not sure which category it falls into for non-native speakers.
While we’re at it with Japanese, how do we define “able to write”? Japanese has at least three different writing systems. Two of them, hiragana and katakana, aren’t too terribly different from how Europeans write: Sure, they’re a larger set of characters, and each of them represents (roughly) a syllable rather than (roughly) a single phoneme, but you’ve still got a fairly small character set each of which has a phonetic value, which you string together into phonetic words. The third, however, kanji, is basically the Chinese writing system, mostly even using the same basic vocabulary, and so mastering it requires memorizing thousands of characters. Do we say that someone can write in Japanese if they can write in hiragana and katakana, but not in kanji? What if they can only write in one of those?
jtur88, it’s not rare for non-native speakers to learn to speak Navajo, and here’s an example:
I don’t know where you got the idea that nobody can learn to speak Navajo fluently if they aren’t native speakers. Perhaps you got it from the nonsense that is sometimes claimed about the Navajo codetalkers. It’s sometimes said that the reason that their code was unbreakable was that nobody except members of the Navajo tribe could understand the language. This is wrong. Not many outsiders tried to learn Navajo back in the 1940’s, but the ones who did (missionaries, anthropologists, etc.) didn’t have huge problems in doing it. Furthermore, the Navajo codetalk wasn’t unbreakable. The Japanese got closer to breaking it than you might expect.
What about some of those obscure languages that only a few people speak. Didn’t Cracked just do an article on those?